Character Analysis
We don't see much of a difference between all of Pemberton's business partners. Sure, one is old and obnoxious (Wilkie) while another is secretive (Harris), but they are all more interchangeable than not. Or, as Serena puts it, "'They're timid men, especially Buchanan. […] Wilkie's just gotten old, but it's Buchanan's nature. The sooner you and I are shed of them the better'" (6.93). To be clear, when she says "shed of them," she means kill them.
It's through the business partners that we get the first glimpse of Serena's gritty, calloused nature. Sure, she wears the pants and talks a big game, but it's not until Buchanan mocks her that we know she can add murderer to her resume.
Over the course of the book all three business partners tick her off, and all three get bested in one way or another. Perhaps their arrogance makes her revenge a little sweeter. Or maybe it's just that Serena doesn't want anyone partnering with her hubby but herself. Whatever the reason, these men become disposable to her—ultimately they are just checkbooks, and she can easily find others.